The present invention relates generally to gas analyzers suitable for use in analyzing a sample gas containing contaminants which normally interfere with the operation of the analyzer and more particularly, is directed to a gas cell for use in such an analyzer.
Both dispersive and non-dispersive radiant energy gas analyzers are used in the prior art to measure certain predetermined constituents of a sample gas which may contain other contaminants which normally interfere with the operation of the analyzer. Applications for non-dispersive type radiant energy gas analyzers that involve measuring constituent gases in a contaminated sample gas involve such diverse applications as breathing gas analysis and determining emission levels in the exhaust of an internal combustion engine. Applications for these types of analyzers in the field of breathing gas analysis include for example the art of capnography, encountered in the medical sciences, and the problem of determining ethanol concentration, encountered in the field of law enforcement.
The art of capnography involves the measurement of carbon dioxide concentrations in breathing gas expelled from a patient's lungs. This data can be useful in determining the patient's ventilatory status and other physiological conditions. The principles of capnography were discovered by K. Luft and were published in 1943 in "Uber eine neue Methode der registrietenden Gasanalyse mit Hilfe der Absorbtion ultraroter Strahlen ohne spektrale Zerlegung," Ztschr.I. Techn. Phys. Vol. 24, 1943, p 97.
Other monitoring techniques exist for monitoring a patient's breathing during anesthesia, however, in addition to being informative, to be useful in an operating room these techniques must be non-invasive, easy for the anesthetist to use, require a minimum of equipment adjustment or calibration; and be relatively free of artifacts. Many of these conventional techniques are notable for their subjectiveness and qualitative character. These techniques include auscultation of breath sound through a precordial or esophageal stethoscope, observation of the breathing movements of a patient's chest or a breathing bag, the color of structures such as the lips and nail beds of the patient and the color of blood from a surgical wound. Quantitative techniques for obtaining a measurement of breathing efficiency involve measurement of the oxygen in the gas mixture delivered to the patient and measurement of the volume of gases exchanged in the breathing circuit. Unfortunately, these methods measure only the volumes and concentrations delivered by the anesthesia machine, and not necessarily those actually received by the patient. It is well known that one of the most common accidents in anesthesia is unrecognized discontinuity in the breathing system. Such a discontinuity can be revealed by oximetry which measures the patient's oxygen saturation, however, instruments of satisfactory quality for conducting such an operation are very expensive and the transducer alone is quite bulky. Highly accurate and comprehensive information on both inspired and expired gases are available by mass spectrometry but this method is even more expensive and bulky, and although direct arterial gas analysis provides a sensitive measure of effective ventilation, its invasiveness limits its applicability.
In contrast, a dynamic breath-by-breath CO.sub.2 analysis with a non-dispersive radiant energy gas analyzer can present many advantages. This method is a sensitive indicator of the adequacy of breathing because of the close relationship between respiratory depth and partial pressures of CO.sub.2 in the arterial blood and in the end expired air. For example, falling CO.sub.2 levels in the patient's breath can indicate when ventilation is increased, the production of CO.sub.2 is decreased, transport to the lungs is decreased, transport between the lips and alveoli is impeded, a gross depression of respiration occurs, transport between the patient and the analyzer is impeded or when there is an admixture of non-CO.sub.2 containing gases. CO.sub.2 levels in the patient's expired gases will increase when ventilation is decreased (other than an extreme decrease), production of CO.sub.2 is increased, transport to the lungs is increased, there is rebreathing, NaHCO.sub.3 is infused or water has entered the analyzing chamber. Although the utility of the method has been recognized for many years, it has not achieved general use because of the limitations of earlier non-dispersive type radiant energy gas analyzers.
Typically, in capnography a non-dispersive infrared gas analyzer is used to measure the concentration of CO.sub.2 in a multi-component gas mixture based on the absorption of an infrared beam of a specific wave length by the CO.sub.2 in the gas mixture. Infrared absorption by a gas mixture is a characteristic of the type and arrangement of the atoms comprising the gas molecules of the mixture. Various types of gas molecules generally exhibit characteristic absorption spectra that are related to the number, configuration and types of atoms in a given molecule. The simpler the molecular structure, the simpler the absorption spectrum. Conversely, the heavy, more complicated molecules exhibit quite complex spectra. Infrared radiant energy intereacts with the molecules of the gas mixture, the degree of interaction being a function of the spectral absorption bands for the different gas components of the mixture and the number of molecules of absorbing gas of each mixture that are present.
By examining the absorption spectra of the specific gas of interest, one can generally locate an infrared absorption band that is unique to that gas or one that at least would not interact with other gases. Typically, the analyzer compares the infrared transmittance of two identical optical paths. One path passes through a sample cell which is filled with a gas of unknown CO.sub.2 concentration and the other path passes through a reference cell which is filled with a reference gas. The spectrum of the infrared beam that is passed through the two cells includes an unique absorption band of the CO.sub.2 molecules within the cells. Since the number of gas molecules per unit volume is proportional to the partial pressure of the gas, a measure of the absorption is a direct indication of the partial pressure of the absorbing gas component. The difference in infrared transmittance between the two optical paths is sensed by a detector whose output is a measure of the partial pressure of CO.sub.2 in the sample. The electrical impulses or output of the detector can also be relayed to a recorder that records a curve on paper that is representative of CO.sub.2 concentration versus time. This curve is known as a capnogram.
Obtaining a representative gas sample of the patient's expired breathing gases is probably the most important consideration and is the source of most potential problems when capnography is attempted. Currently the best way to obtain a sample gas is to modify the breathing head on the ventilator circuit to accept an analyzer sampling line. The sampling catheter should be attached so that it draws from the expired air stream. Other techniques employ an adapter that fits between a ventilator and an endotrachial tube for obtaining a gas sample. A more complete description of the art of capnography may be found in "Capnography In Infants and Children" by Phillip P. Nuzzo, Respiratory Therapy, September October, 1978.
Problems with prior art non-dispersive type CO.sub.2 gas analyzers that have prevented their widespread use in the operating room have included inconvenience of operation due to the complexity and bulkiness of the equipment; the difficulty of obtaining representative end-tidal samples from the anesthetized patient (end-tidal refers to the very last portion of the breath exhaled by a patient) especially at small tidal volumes and rapid respiratory rates; instability of the analyzer itself requiring frequent zero adjustment and recalibration; the high incidence of artifacts due for example, to interference from N.sub.2 O, electrocautery and particulate matter such as water, blood or mucous in the sample cell; and lastly, the high cost of the analyzing equipment.